This implements support for calls to and from C in the ppc64 C ABI, as
well as supporting functionality such as an entry point from the
dynamic linker.

Change-Id: I68da6df50d5638cb1a3d3fef773fb412d7bf631a
Reviewed-on: https://go-review.googlesource.com/2009Reviewed-by: Russ Cox <rsc@golang.org>